Electric wave system



June 6, 1944. M K, zlNN 2,350,951

' ELECTRIC WAVE SYSTEM Filed 001'.. 51. 1941 Patented June 6, 1944 y ELECTRIC WAVE SYSTEM Manvel K. Zinn, Manhasset, N. Y., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y.. a corporation of New York Application October 31, 1941, Serial No. 417,331

24 Claims.

The invention relates to electric wave systems. and, more particularly, to a transmissionsystem including a submarine cable having one or more repeaters or amplifiers inserted therein at spaced intervals between its ends.

Anobject of the invention is to improve submarine signal systems.

Another object is to enable regulation of the gain of an amplifier in a submarine cable in a simple and eillcient manner. l

A further object is to enable selective regulation of the gain of a plurality of amplifiers ipserted'in a submarine cable at spaced intervals between, the shore ends thereof.

In accordance with this invention these and still other objects are realized in a transmission system comprising a transmission line, specically, a submarine cable, having a plurality of amplifiers inserted therein at spaced intervals between the ends thereof, the signal current to be amplified and the' energizing current for the electronic devices in the amplifier being transmitted over the line from one end thereof. Each amplifier includes a stabilizing negative feedback connection incorporating a temperature-dependent resistance element or thermistor, variation in the resistance of which is adapted to vary the gain-reducing effect of the feedback connection.. The temperature and, therefore, the resistance of the thermistor, is dependent on the magnitude of the energizing current supplied over the line, the energizing current nor- Imally being of a substantially fixed or constant value. The gains of all of the amplifiers may be simultaneously adjusted by a change in the magnitude of the energizing current, or the lgains of some or all of the amplifiers may be changed to the same or different degrees by superposing on the substantially constant energizing current a low frequency control current which may be an electric wave of sinusoidal or of the square topped type. The low frequency current may be of the order of a fraction of a cycle per second or of the order of a few cycles per second, and may have a wave-length equal to or approximating the length of the transmission line.

A more complete understanding of the invention will be obtained from the detailed description that follows, taken in conjunction with the appended drawing, wherein:

Fig. 1 represents a submarine cable or transmission system for signal transmission` in one direction between two widely separated points;

Fig. 2 shows in detail the circuit configuration of a repeater or amplifying station included in the cable of Fig. 1 at spaced intervals:

Fig. 3 shows the volts-resistance characteristic curve for a thermistor circuit included in the amplifier of Fig. 2 for gain control purposes; and

Fig. 4 shows the attenuation and wave-length versus frequency characteristics of a submarine cable over a portion of the low frequency range.

With reference in particular to Fig, 1, there is shown a submarine cable I0, connecting two land points A, B, for transmission of signal currents in one direction, for example, from point A to point B. This cable may be of the general type disclosed in O. E. Buckley-O. B. Jacobs Patent No. 2,020,297 of November 12, 1935, in that it encloses a plurality of carrier frequency signal repeaters or amplifiers I, 2 n at spaced intervals; power supply for the amplifying devices at the amplifiers being furnished over the cable from suitable sources II, II', for example, batteries, at the ends of the cable. In accordance with a feature of the invention, a source I2 of very low frequency alternatingcurrent may be connected in the power supply path at one end of the cable. The source l2 may comprise a suitable alternating current generator; or it might comprise a variable resistance caused to increase and decrease cyclically between preass'igned limits at a desired frequency, whereby the current supplied to the cable fromthe power supply contains a low frequency component. A low pass lter I3 may be inserted between the cable and the sources II, Il' to confine the signal currents to the cable and the input and output transformers I4, I5 at the ends of the cable.'

Fig. 2 is illustrative of the repeaters or amplifiers I, 2 l. n enclosed at spaced intervals in the cable I0. Each amplifier may comprise a plurality of electronic devices, specifically, vacl uum tubes 20, connected in tandem and comprising an indirectly heated cathode, an input grid, a screen grid, a suppressor grid and an anode. The heater filamentsfor the tubes are indicated by the numeral 2|. The interstage networks 22, 23 may -be transformers, the initial electronic device ybeing connected with the cable through an input transformer 24 and the final stage with the cable through output transformer 25. The amplifier circuit may be of the type that includesv a stabilizing negative feedback connection or path 26 of the general type proposed. in H. S. Black Patent No. 2,102,671 of December 21,I 1937. A temperature-dependent variable resistance or thermistor'T1 is connected resistance, for example, uranium oxide, which has a high negative temperature coefllcient of resistance. The thermistor Ti, as explained in greater detail hereinafter, is provided for amplifier gain control purposes, andeis adapted to be heated indirectly by a heater coil 21 connected in series with a second thermistor Tz across a resistor 28. The resistor 28 is connected in series with the heater filaments 2i in the power supply connectionor loop 2l around the ampliiler. Anode and screen grid potentials for the electronic devices ar'e provided over the cable and connection 30 in the amplifier, the necessary difference of potential between anode and cathode being obtained by connecting the latter to the point of lowest relative potential in the amplifier, and the anode to the point of highest relative potential. The anode and screen potentials therefor are provided by the potential drop across the heaters 2| and incidental reisistances in the power supply loop at the amphiler,y the direction of power supply current now being assumed from left to right in. Fig'. 2. Lowpass filters comprising coils Il and condensers 32 segregate the signal currents channel and the power supply loop 29.

'I'he arrangement described permits of remote control, that is, from one end of the transmission system, of the gains oi amplifiers enclosed in a submarine cable. 'Ihe cable with its self-contained amplifiers would, of course, be laid with the expectationv of not doing any maintenance on the ampliers except in the event of a disabling fault requiring a major repair.

Automatic or manual gain control arrangements are usually included in transmission systems intended for use either in the open air or buried in the earth because of the effect of temperature variations on the attenuation char:

acteristic of the transmission line embodiedl in,

the system. This problem does not exist with submarine transmission systems, particularly in the case of a cable on a ocean bottom, since the temperature thereat is substantially invariable. For one or more of the following reasons, it may be desirable, however, to be able to control from the shore terminals the gain of the amplifiers embodied in a system such as has been described with reference to Figs. 1 and 2: The amplier gain will decrease slowly with time because of ageing of the velectronic devices. Unavoidable small errors in design calculations for thetsystem may make initial gain adjustment necessary. In testing the submarine cable after it has been laid and particularly in trying to locate an ampliiler that has developed a fault, a method and means of changing the amplier gain would be useful. Because of noise conditions resulting from unanticipated disturbances or originating in noise sources of a temporary character, it may be desired to change the gains of some of the amplifiers by an amount different from that of the others.

In accordance with one feature of the invention the gains of all of the amplifiers may be changed simultaneously by changing in a small amount the magnitude of the direct current supplied over the cable from the sources Il, Il'. This change in the magnitude of the direct current may be effected by adjustment oi' the variable resistances 40, 40'. 'I'he change in the direct current could be too small to have any adverse effect upon the 75 of the cable,

operation of the electronic devices. but yet be sumcient to cause a change in the resistance of the thermistor T1, the temperature of which is controlled by the potential drop in the resistor 28. Variation in the potential drop across the resistor 28 causes a variation in the heating effect of the series-connected heater coil and the thermistor Ta on the thermistor T1. The thermistor Tz, it will be noted, is directly heated, and in troduces a booster or amplifier eiiect with respect to the heating current flowing in the heater coil. Although the thermistor Ti is shown connected in shunt in the feedback path of the amplifier, it could be connected in the series path of the feedback connection; the shunt arrangement is preferable, however, because failurey of the thermister Ti because of ageing or burning out merely changes the gain of the amplifier by a small amount, whereas, if the thermistor were connected in the series path of the feedback connection, the amplifier would be disabled. Variation in the temperature and, therefore, in the resistance of the thermistor T1 varies the amount of loss in the feedback connection and, consequently, causes a change in the over-all gain of the amplifier. Fig. 3 s hows a characteristic curve for a typical thermistor circuit such as is included in the amplier of Fig. 2. Along the horizontal axis is plotted voltage across the series-connected heater coil 21 and thermistor Tz, and along the 'vertical axis is plotted the resistance of the thermistor Ti. If the direct current supplied over the cable were .25 ampere, and the resistance of the resistor were l0 ohms, 2.5 volts would normally be impressed on the thermistor circuit, under these conditions the thermistor T1 would have a resistance of 200 ohms. If the beta circuit lmpedance were of the order of, say, 250 ohms pure resistance, the .200 ohms thermistor would then introduce a loss of 4.2 decibels in the feedback path. A change as small as i2 per cent in the direct current would cause a change of voltage across the resistor 2l of from 2.55 to 2.45 volts and this in turn would cause a change in the resistance of the thermistor T1 from 170 to 230 ohms. Such changes in the resistance of the thermistor Ti would vary the loss in the beta circuit from 4.8 decibels to 3.8 decibels. Since, in feedback ampliers as generally designed, the change in gain .obtained is substantially equal to the change in the loss in the beta circuit, the gain of the amplifier would be'changed over a range of extent than that of others in the cable. This may be done by superimposing a slowly varying alternating current on the normal steady supply current, for example, by means oi' ythe device i2. Since the variable component of the total current would be attenuated and the steady component would not, the effective value of the total current would decrease along the cable. As a result, the gains of the amplifiers nearest the control terminal would be affected more than would those at more distant points. By sending the variable component of current from one end orthe other the ampliilers at either end could If it is assumed that the ampliilers the more sensitive ones in the amplifiers near the middle of the cable, so that a given change or variation ln the power supply current produces more change in gainv in the amplifiers near the middle of the cable than in those near the end. Any mode of gain variation along the cable between this condition and the condition where all the gain changes are alike may be obtained by superimposing an appropriate magnitude of slowly varying alternating current at the end of the cable to compensate forthe lesser sensitivity of the thermistor circuits inthe amplifiers nearest the ends of the cable.

To obtain an appreciable change in the effective value of the total current sent through the power supply loop of the amplifiers may require an alternating current of appreciable magnitude. If the normal or steady value of the power supply current is Io, and the maximum amplitude of the sinusoidal current is n, the effective total current is I I 1 I, 2 (2) 1- V lidia] This indicates that if the superimposed sinusoidal current has an amplitude of one-tenth that of the steady current, the effective value of their combination would be of the order of one per cent greater than the steady current. To obtain a 2 per cent increase in the total effective current requires that the amplitude of the varying current be made about 28 per cent that of the steady current. The desired result, however, could be obtained with smaller peak current uctuations by using a square topped wave instead of a sinusoidal wave. Such a square-topped wave could be fgenerated, .of course, by means of a simple -commutator arrangement. For a superimposed square-topped wave of amplitude In. the formula for the eilective'value of the total current would be I 4 1 2 ITV 1+(r2) To obtain a 2 per cent increase in eiective current would then require that In be only per cent of. Io.

Fig. 4 shows an attenuation vs. frequency and a wave-length vs. frequency characteristic curve for the power supply channel of a submarine cable of the type disclosed herein and having the constants indicated in the figure. channel is essentially a heavily loaded" circuit with a low cut-off frequency, the"loads" being the lter coils 3| at each amplifier. It will be noted that the attenuation is substantial, and increases very rapidly with increase in frequency. By way of example, it will be observed from the attenuation-frequency curve that for a current varying at the rate of say one cycle per second, the attenuation is of the order of .0185 decibel per mile, or t18.5 decibels for a thousand miles, which is about half the length of a transatlantic cable. The eflect on the ampliiler gains of superimposing a one cycle per second current on the normal steady current, would be i'eltv only close to the end of the cable. For a superimposed cur- At the middle of the cable the total current would then be increased by the factor 1 (.3555)I .1266a v If all of the thermistor circuits at the amplifiers were alike, the changes in gainsof the am- The power pliiiers at the middle-of the cable would be about one-eighth of those at the control end of the cable. Conversely, to obtain approximately uniform gain-changes under these conditions would require that the thermistors at the middle of the cable be eight, times as sensitive relatively. v

The use of superimposed slowly varying alternating current for selective gain control would l not cause the amplifier gains to go up and down appreciably with the variation because'of the stabilizing effect of`feedback. In addition, the modulation envelope produced at successive amplifiers would be out of phase and the ripple would tend to average out.. The thermistor T1 at each ampliiier should be proportioned so as to have suiiicient lag in heat transfer so that its resistance does not follow the slow variation in the power supply current.

Although this invention has been disclosed with reference to certain specific embodiments, it will be-understood that it is not limited thereto, but

that its scope is evidenced bythe appended claims. i

What is claimed is:

1. A- signaling system comprising a transmis- Y is supplied from an end of the cable to a power" consuming amplifying device in said cable between the shore ends thereof, and means for superimposing on said current an alternating current of wave-length of the order of that of the length of the cable for regulating the gain of said amplifying device.

3. In a transmission system comprising a transmissionvline and an ampliilerY inserted in the line between the ends thereof and in which both the signal current to be ampliiied and the energizing potential and current for the amplifier are supplied to the amplier over the line, means to superimpose on said energizing current a low vfrequency alternating current and means in said marine cable andan amplifier inserted inthe cable between the shore ends thereof and in which both the signal current to be amplified and the energizing potential and current for the amplifier are supplied to the ampliner over the cable, means tosuperimpose on said energizing current a low frequency alternating current and means in said amplifier to vary the gain of 'the amplifier, 'said means being responsive to said lowl frequency alternating current. y l

A signaling system comprising a transmission line over which a substantially constant current is supplied from one end of the line to a power consuming amplifying device in said line,

l and means for introducing a low frequency -cornponent in said current for regulating the gain,

amplifier energizing current over said line from one end thereof, said energizing current comprising a direct current component and a aubaudible '1. A signaling system comprising a submarine devices inserted in said cable at spaced intervals between the shore ends thereof, and means for introducing a low frequency component in said current for selectively regulating the gain 1of said devices.

9. A signaling system comprising a transmission line, a plurality of signal current amplifiers inserted therein at spaced intervals between the ends thereof, a gain reducing feedback connection in each amplifier, a thermistor in the feedback connection for varying the gain reducing eect of the feedback connection, each amplier havingv a signal current path therethrough and a power supply path therearound, means at one end of low frequency component.

12. A signaling system comprising a transmission line, a signal current ampliner inserted therein between the ends of the line, a gain reducing feedback connection in said amplifier, a thermistor in the feedback connectionforvarying the gain reducing eifect of the feedback ccnnection, said amplifier having a signal current path therethrough and a power suppl! path therearound, means at one end. of the lineffor supplying a substantially constant current over said line through the lpower supply path at the amplifier to provide the required energizing potential and current to the amplifier and itl thermistor, and-means to introduce a lowvfrequency component in said power supply'current to regulate the temperature of the thermistor.

13. 'Ihe system of claim 12 in which the power supply path around the ampliner includes means for indirectly heating the thermistor, said heating means including a second thermistor.

14. A signaling system comprising a transmission linel over which a substantially constant current is supplied from one end of the line to a the line for supplying` a substantially constant supply current to selectively regulate the temperature of the thermistors.

10. A signaling systemcomprising a submarine cable, a plurality of signal current ampliers inserted in the cablezat spaced intervals between the shore ends thereof, a gain reducing feedback f connection in each amplier including a thermistor for varying the gain reducing effect of the power consuming device in said line. said powerV consuming device including a thermistor, means for transmitting periodically varying current over said line and means for causing said periodically varying current to regulate said device through variation of the temperature of said thermistor.

15. A signaling system comprising a transmission line over which asubstantially constant current is supplied from an end of the line to a power consuming amplifying device in said line. said amplifying device including a gain regulating circuit containing a thermistor, means for transmitting periodically .varying current over said line' and means for causing said periodically varying current to regulate said gain circuit through variation of the temperature of said thermistor.

16. A transmission system comprising a transmission line and an amplifier inserted in the line between the ends thereof and in which both a signalcurrent to be amplified and the energizing potential and current for the amplifier are supplied to the amplifier over the line. means to superimpose a low frequency alternating current on said energizing current and a thermistor in said amplifier to vary the gain of the ampliner. said thermistor being responsive to said low frequency alternating current.

feedback connection, each amplifier having a signal current path through the amplifier and a power supply path around the amplifier, means at a shore end of the cable to supply a substantially constant current over said cable and through the power supply path at each amplifiery to furnish the required energizing potential and current to each amplifier and thermistor, and means to introduce a low frequency component in said power supply current to selectively regu- 17. In a transmission system comprising a submarine cable and an amplifier inserted in the cable between the shore ends thereof and in which both the signal current to -be amplified andthe energizing potential and current for the amplifier are supplied' to the amplifier over the cable, means to superimpose a low frequency alternating current on -said energizing current and a thermistor in said amplifier for varying the gain of the amplifier, said thermistor being responsive to said llow frequency alternating current.

18.1A signaling system comprising a submarine cable over which a-substantially constant current is supplied from one shore end of the cableto a plurality of power consuming amplifying devices inserted in said cable at spaced intervals between the shore ends thereof, a thermistor in each of said amplifying devices, and means for introducing a low frequency component in said current forselectively regulating the gains of said devices through variation in the temperatures ot thermistors.

19.*., In a transmission system comprising a power consuming device lat a distance from the l source of power supply for said device, the method said 21. In a transmission system comprising a power consuming device at a distance from the source o power supply for said device, the method that comprises transmitting a direct current from said source to said device to energize the latter and introducing a square-'topped wave component in said direct current for regulating the power level of said device.

22. .The method of claim 19 in which said alternating current component is of subaudible frequency.

23. The method of claim 20 in which said sinusoidal component is of subaudible frequency.

24. The method of claim 21 in which said square-topped wave component is of subaudible frequency. i MANVEL K. ZINN. 

